The present embodiments relate to a method for the manufacture of doped quartz glass. Moreover, the present embodiments relate to quartz glass obtainable according to the method and to the use thereof in the field of optics, for example as optical component.
The incorporation of foreign atoms into quartz glass allows the properties thereof to be affected. Accordingly, the doping of quartz glass with fluorine decreases the refractive index. Fluorine-doped quartz glass is therefore used, inter alia, for the manufacture of light-guiding refractive index structures in optical fibers, for example bending-insensitive fibers or in so-called “ultra low loss fibers”. In this context, a number of methods are available to a person skilled in the art. Accordingly, a pre-mold that has a refractive index profile in radial direction and can be drawn directly to form the desired fiber can be used as semi-finished product for the optical fiber. Alternatively, a rod- or tube-shaped cylinder comprising at least one layer made of fluorine-doped quartz glass can be used. It can be elongated to form the fiber in an assembly together with other cylindrical components in coaxial arrangement. The fluorine-doped quartz glass cylinders are also used in laser and semi-conductor production.
Usually, the doping materials are rare earth elements, such as yttrium, and transition metals, such as aluminum and titanium, which are to attain the highest possible amplification performance. The amount of foreign ions that can be introduced into the quartz glass is limited though, since the presence of the doping agents in the quartz glass changes the refractive index thereof, which might lead to undesired side effects. In order to counteract these challenges, the quartz glass is additionally doped with fluorine, which is known to lower the refractive index of quartz glass. However, this also is associated with some production-related restrictions, which limit the amount of fluorine that can be introduced into the quartz glass and make a homogeneous distribution difficult to attain, especially if the wall thickness is high.
The introduction of foreign atoms is further complicated by hydroxyl groups (OH groups) being present in the soot body and needing to be removed during the manufacturing procedure, for example through various steps of drying. This, in turn, is associated with the problem that these need to be compatible with the doping methods. The prior art describes various approaches to a solution.
DE 102 18 864 C1 describes a method for the manufacture of a cylindrical quartz glass body having a low OH content, in which the soot body is subjected to a dehydration treatment.
WO 03/101900 A1 discloses a method for the manufacture of a doped optical fiber pre-mold, in which the soot body is first treated in a chlorine-containing atmosphere and is subjected to a fluorine-containing gas in a subsequent step.
EP 0 161 680 describes a method for the manufacture of a glass pre-mold for an optical fiber, in which the glass pre-mold is formed from fine glass particles containing SiO2 and is then sintered in an atmosphere of He and SiF4.
DE 10 2005 059 290 A1 relates to a method for the manufacture of a cylindrical form body made of transparent quartz glass through layer-by-layer deposition of SiO2 particles on a substrate while forming a porous soot body. The soot body is then subjected to a negative pressure sintering treatment.
EP 0 139 532 B1 describes a method for the manufacture of a pre-mold made of glass for optical fibers by heating a soot-like glass pre-mold in the presence of a fluorine-containing gas.
In some examples, it has proven to be advantageous to vitrify the soot body at reduced pressure. As a result, the sintering process is associated with concurrent dehydrogenation of the soot body and any inclusions are prevented. Moreover, the formation of bubbles in the later quartz glass is minimized. However, the method is associated with a disadvantage in some instances in that some of the doping agents that are physically bonded in the soot body, in particular gaseous fluorine compounds, might desorb during the vitrification process, in the outer layers. This results in the formation of an undesired concentration gradient and fluorine depletion.
One way of counteracting fluorine depletion, in particular in the outer layers of the soot body, is to vitrify in a fluorine-containing atmosphere. This ensures that a sufficiently high partial pressure of fluorine-containing compounds, such as, for example, SiF4 or HF, continues to be present in the gas phase during the vitrification such that no removal of fluorine from the soot body takes place during the vitrification. However, the conventional methods are disadvantageous in that the furnace materials, in which the vitrification in the presence of a fluorine-containing glass is performed, need to meet strict requirements. Due to the presence of highly reactive media, such as SF4, or derived products, such as HF, in combination with the high process temperature, the furnace materials are exposed to extreme conditions. This causes strong wear and tear, which again results in high maintenance and up-keeping costs. In addition, the extreme requirements also limit the batch size as both the requirements and the stress on the furnace materials increase with increasing furnace size. Moreover, the selection of furnace materials is very limited, since these need to have not only high temperature and corrosion resistance, but also high purity. Typical furnace materials, in which vitrification can take place in an aggressive atmosphere, include, for example, inliner pipes made of quartz glass, but these are associated with a disadvantage in that they change shape over time. The higher the vitrification temperature, the larger is the wear and tear on the materials.
Moreover, vitrification in a gas atmosphere is associated with an elevated risk of incorporating bubbles into the glass, which has an adverse effect on the optical properties of the quartz glass.
There is therefore a need for a process that allows for vitrification of fluorine-doped soot bodies in a vacuum without concurrent formation of a concentration gradient of the dopant. Instead, high and homogeneous fluorine doping is to be attained.